Scientists announced Thursday the discovery of three planets that are some of the best candidates so far for habitable worlds outside our own solar system - and they're very far away.
NASA's Kepler satellite, which is keeping an eye on more than 150,000 stars in hopes of identifying Earth-like planets, found the trio. Two of the planets - Kepler-62e and Kepler-62f - are described in a study released Thursday in the journal Science. They are part of a five-planet system in which the candidates for life are the farthest from the host star.
The host star - the equivalent of Earth's sun - takes the name Kepler-62, where the individual planets are designated by letters thereafter. A third planet that's potentially habitable, but not included in the Science study, is called Kepler-69c. These are the smallest planets ever found in the "habitable zone," the area near a star in which a planet can theoretically hold liquid water.
Kepler-69c seems less clearly in the habitable zone than the other two planets, but scientists haven't ruled it out. "With all of these discoveries we're finding, Earth is looking less and less like a special place and more like there's Earth-like things everywhere," said Thomas Barclay, Kepler scientist at the Bay Area Environmental Research Institute in Sonoma, California.
You won't be swimming on the planets anytime soon, though. The Kepler-62 star is 1,200 light-years away; Kepler-69 is 2,700 light-years away. A light-year, the distance that light travels in a vacuum in one year, is nearly 6 trillion miles.
What are these planets like?
The smaller a planet is, the more likely it's rocky and the less likely it's made of gas, said William Borucki, Kepler science principal investigator at NASA Ames Research Center. That makes Kepler-62f, thought to be 40% larger than Earth, potentially the most like our planet out of the new discoveries.
It could be rocky, Borucki said, with polar caps, land mass and water as well. It goes around its star once every 267.3 days (Earth days, that is). If you were standing on Kepler-62f, the star in the sky would look bigger than our sun does, but "the illumination level would be like walking around on Earth on a cloudy day," Borucki said at a press briefing.
"It drops by a factor of five." Kepler-62e appears to be 60% larger than our planet and a little closer to its host star; this one could be a "water world" of mostly deep oceans, he said. It circles its star in 122.4 days. "All these planets that we're finding are quite different than planets in our own solar system," Borucki said.
Kepler-69c appears to orbit a star similar to Earth's sun, Barclay said. As it's estimated to be about 70% larger than Earth, it may also be a water world, with oceans thousands of kilometers deep. This planet is also exciting because it is the smallest scientists have found that orbits a sun-like star in the habitable zone. It's not likely to have a rocky surface, Barclay said.
According to what little we know about it, Kepler-69c is probably significantly warmer than Earth and could be more like Venus. "Probably, if there is life, it would be very unlike what we see on our own world," Barclay said. If scientists' notions of a "habitable zone" were applied to our own solar system, both Earth and Mars would fit the bill.
But Mars doesn't have enough gravity to hold onto an atmosphere that could heat it sufficiently, said Lisa Kaltenegger, research group leader at the Max Planck Institute for Astronomy in Heidelberg, Germany, in a press conference. Still, the Mars rover Curiosity has uncovered evidence that the Red Planet was, indeed, hospitable to life in the past
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